Editing Atom (section)

=== Formation ===
{{Main|Nucleosynthesis}}
[[File:Nucleosynthesis periodic table.svg|thumb|600px|Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the [[alpha process]]. Elements beyond iron are made in large stars with slow neutron capture ([[s-process]]). Elements heavier than iron may be made in neutron star mergers or supernovae after the [[r-process]].]]

Electrons are thought to exist in the Universe since early stages of the [[Big Bang]]. Atomic nuclei forms in [[nucleosynthesis]] reactions. In about three minutes [[Big Bang nucleosynthesis]] produced most of the [[helium]], [[lithium]], and [[deuterium]] in the Universe, and perhaps some of the [[beryllium]] and [[boron]].<ref name=ns1794_42 /><ref name=science267_5195_192 /><ref name=hinshaw20051215 />

Ubiquitousness and stability of atoms relies on their [[binding energy]], which means that an atom has a lower energy than an unbound system of the nucleus and electrons. Where the [[temperature]] is much higher than [[ionization potential]], the matter exists in the form of [[plasma (physics)|plasma]]—a gas of positively charged ions (possibly, bare nuclei) and electrons. When the temperature drops below the ionization potential, atoms become [[statistical physics|statistically]] favorable. Atoms (complete with bound electrons) became to dominate over [[charged particle]]s 380,000&nbsp;years after the Big Bang—an epoch called [[recombination (cosmology)|recombination]], when the expanding Universe cooled enough to allow electrons to become attached to nuclei.<ref name=abbott20070530 />

Since the Big Bang, which produced no [[carbon]] or [[atomic number|heavier elements]], atomic nuclei have been combined in [[star]]s through the process of [[nuclear fusion]] to produce more of the element [[helium]], and (via the [[triple-alpha process]]) the sequence of elements from carbon up to [[iron]];<ref name=mnras106_343 /> see [[stellar nucleosynthesis]] for details.

Isotopes such as lithium-6, as well as some beryllium and boron are generated in space through [[cosmic ray spallation]].<ref name=nature405_656 /> This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected.

Elements heavier than iron were produced in [[supernova]]e and colliding [[neutron star]]s through the [[r-process]], and in [[Asymptotic giant branch|AGB stars]] through the [[s-process]], both of which involve the capture of neutrons by atomic nuclei.<ref name=mashnik2000 /> Elements such as [[lead]] formed largely through the radioactive decay of heavier elements.<ref name=kgs20050504 />